The present invention relates generally to an electromechanical reed filter, particularly to an electromechanical reed filter of a miniature type employing vibratory reeds as frequency filtering elements, and more particularly to means for ensuring stability of the operation of the filter against external shocks being imparted to the filter.
It is well known that a tuning fork which consists of a pair of vibratory reeds (usually made of metal) extending from the end of a stem, when struck, emits a tone of fixed pitch. Owing to their simple mechanical structure, purity of tone, and resonant frequency, tuning forks are widely used as standards of frequency in the acoustics. Furthermore, because of the constant frequency which is primarily determined by the length and the thickness of the reed elements, tuning forks are also used in electronics as a filtering element in an arangement known as an electromechanical reed filter. In such an arrangement, the tuning fork is provided with a pair of electromechanical transducers such as piezoelectric elements, one for causing vibrations by an exciting input signal and the other for converting the vibration at the natural frequency of the tuning fork into an electric signal. The tuning fork, when excited by the input signal, is caused to vibrate at its natural or fundamental frequency, and thus only the signal having a frequency which is equal to the fundamental frequency is obtained at the output. In the filter arrangement, the piezoelectric elements are usually adhered or bonded by an adhesive to the side surface of the respective reed elements and coupled to input and output circuits.
The size of the reed elements is however intimately associated with the frequency desired, and thus relatively large as compared with other electronic device components. Therefore, the tendency is toward reducing the size of the casing in which the reed elements are housed, so that very slight clearance will be allowed between the inner walls of the casing and the reed elements of the tuning fork. Under such circumstances, violent transverse vibrations would occur at the free ends of the reeds upon the filter casing being subjected to abnormal external shocks which cause a flexure of reed elements to a considerable degree and the free ends of the reeds would collide violently with each other and/or with the inner walls of the casing. For this reason, deformation and/or cracks are likely to occur especially in the adhesive securing the piezoelectric element to the reed element and the contact therebetween in severely impaired. Due to these undesirable occurrences, the accuracy of resonance of the filter is forever lost. In addition, the incipient cracks spread rapidly and cause permanent damage to the filter.
Therefore, shock-absorbing means have been proposed to effectively remove the aforementioned defects inherent in the conventional electromechanical reed filter of a miniature type. However, they have been proven to be unsatisfactory for effectively absorbing external shocks as will be discussed in detail in connection with FIGS. 1a and 1b.